Vernier Calipers And Micrometer Screw Gauges, Measuring Without Compromise
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Vernier Calipers And Micrometer Screw Gauges, Measuring Without Compromise

Jul 07, 2023

I needed a temperature controller module recently, so off I went to Banggood to order one. As one does I found myself browsing, one thing led to another, and I bought a micrometer screw gauge. While micrometers are pretty expensive devices, reflecting their high precision engineering and construction, this micrometer cost me only about £8, or just under $10, definitely in the spirit of our long-running series of reviewing very cheap tools in search of a diamond in the rough. But perhaps more importantly, this is also the cue for an examination of high precision dimensional measurement. So I’ve assembled a collection of micrometers and vernier calipers of varying quality, and it’s time to dive in and measure some very small things.

Some of you will be metrology enthusiasts with an array of the finest devices available, but I am guessing that many of you will not. The ubiquitous precision measurement device in our community appears to be the digital caliper, a sliding clamp with an LCD display, an instrument that can be had in its most basic form for a very small outlay indeed. For the purposes of this piece though we’re not looking at digital devices but their analogue precursors. If you want a feel for metrology and you’d like some of those heritage tools that parents pass onto their kids then it’s time to learn something about the vernier caliper and the micrometer.

A Vernier caliper is a graduated sliding clamp usually over 150mm long, with a Vernier scale that allows the reading of fractions of its graduation. It will typically have two sets of jaws for both internal and external measurements, and an extending probe for depth measurements. On the body of the clamp are the millimetres or inches, and on the sliding jaw is a scale graduated in a constant fraction of the main scale. It’s a mathematical trick that allows the user to turn “ten and a bit millimetres” into “10.12 mm”, by working along the Vernier scale until one of the gradations lines up with a gradation on the main scale, and reading the fraction of a millimetre figure from the Vernier scale.

I have a friend with a background in a family business grinding tungsten carbide to a very high precision indeed who jokingly refers to a vernier caliper as “the guessing stick” because its precision is several factors of ten above her working zone, but the ability to add an extra digit and fraction of a digit’s precision to a millimetre reading depending on the Vernier scale in question over the whole relatively long range of the caliper makes it a very useful instrument indeed.

A micrometer is a G clamp with jaws in its most common form around 25 mm in size, and with a very accurately calibrated screw thread. It has a handle with both a scale in millimetres or inches along its length and a fractional scale on the handle that turns the thread. Using it is a case of closing it on the item to be measured, reading the millimetres from the length scale, and the fractions of a millimetre from the handle scale. A typical micrometer has a precision of a hundredth of a millimetre or a thousandth of an inch, but there are micrometers that combine the two micrometer scales with a Vernier scale round the shaft of the instrument to give an extra fraction of a digit’s precision on top of that. All precision metrology instruments should be taken care of to preserve their calibration, and the micrometer has a ratchet on its end to ensure a constant torque when it is tightened. It should always be tightened using this ratchet, to protect the thread from wear and distortion.

I’ve amassed a selection of both for comparison. In the caliper corner are a very cheap (around £5, or $6) digital caliper, another very cheap plastic analogue Vernier caliper, and my Mitutoyo 530-122 Vernier caliper. The former two are my quick measurement tools of choice, when you see a product dimension in one of my Hackaday pieces it will have been measured on one of them. The plastic Vernier caliper has been with me for about twenty years, for its £1 ($1.50 in 2000) purchase price it has been probably the best values I have ever had in a tool. The Mitutoyo is their entry-level model, I seem to remember it cost me around £30 (about $37), and it comes out of its box when I need a precision measurement that I can really trust.

The plastic calipers are both insubstantial, being slightly flexible and lightweight, with jaws that don’t quite meet along their whole length and which have a discernible slight play on them. The digital one claims a 0.1 mm precision while the Vernier one claims a 0.05 mm precision, which I can imagine might be optimistic in both cases due to those jaws. It’s a case of evaluating them based on their use case and cost though, so as long as I am using them as instruments that can knock around in my drawer to take quick millimetre measurements rather than machining nuclear reactor parts, then they are adequate for that task. By contrast it is immediately apparent on picking up the Mitutoyo caliper that it is a much higher quality instrument, being machined from stainless steel it has a much more substantial weight. The overwhelming impression is one of solidity, the jaws meet perfectly and have no discernible play, and the sliding action is smooth but with a constant slight resistance. It has a precision of 0.02 mm, which is considerably more believable than the figure on the plastic calipers.

Having examined the calipers, how do they perform? It’s time to put them through their paces by comparative measurement. I’ve taken some of the items I have to hand, a sheet of printer paper and a bare badge PCB. It’s important to note that all the measurements on this page are the result of repeated tries to ensure a consistent reading, with these instruments it’s easy to secure a poor reading by misaligning the jaws over the item to be measured.

Straight away the lower resolution of the cheap digital caliper becomes obvious, as it resolves the copper layers on a PCB to be a whopping 0.1 mm. It’s a surprise that it doesn’t resolve the 0.1 mm sheet of paper as such, but this is likely to be an error induced by the play in the jaws. Likewise the plastic Vernier caliper gives a significantly different reading from the Mitutoyo, probably also due to its inconsistent meeting of its jaws.

Meanwhile aside from my £8 micrometer I’ve borrowed a couple of considerably higher quality instruments, a Mitutoyo 103-137 metric model which sells for about £57 ($70), and a vintage 1960s Moore & Wright No. 964 Imperial model. The latter is no longer made but is I think equivalent to current M&W models costing over £50 ($61). They are both general purpose 0 – 25mm/0 – 1″ workshop micrometers, and their owner has taken care of them so they are in very good condition.

The £8 Daniu-branded micrometer is substantially built, with an enamel-painted drop-forged frame and a machined aluminium micrometer head and ratchet. All is not well in its machining however, as I can see a sliver of light indicating imperfect flatness between its jaws when they are closed, and there is discernible play in its thread. It has a 0.01 mm resolution, but given the not-entirely-flat jaws and the play in the thread I would be inclined to add a significant error bar to this.

By comparison both the Mitutoyo and the Moore & Wright have very similar drop-forged frames and machined aluminium heads, but it soon becomes evident where the extra money has been spent. There is no discernible play in either instrument, and their action is smooth with a slightly greater resistance than the cheaper one. The faces of their jaws have been lapped to a mirror finish, and there is no discernible gap between the jaws when they are closed.

Looking at the performance of the micrometers I’ve yet again performed a series of measurements with each. The sheet of paper has been replaced with a hair plucked from my own head, in homage to that lesson in engineering elementary school when you’re told a micrometer can measure a human hair.

Straight away we can see that the Mitutoyo and Moore & Wright agree with each other pretty closely, while the Daniu differs significantly on some readings. The play in the thread and not entirely flat jaws make consistent readings difficult, as the reading depends on the alignment of jaws and thread at the distance in question.

Having examined both decent and cheap Vernier calipers and micrometers, I’ve established that a decent quality instrument beats a very cheap one. My plastic calipers are fine for the quick millimetric measurements I use them for but maybe not for higher precision work. Unsurprisingly an £8 micrometer doesn’t deliver consistent readings at its claimed resolution but would be adequate at a digit’s less precision. The real story lies in how useful the quality instruments can be even in the age of digital metrology, and that the starting price for a decent one perhaps isn’t as high as you might think.

There’s also another point which is a little frivolous but with which I suspect more than a few readers will identify, and that’s the feel of a quality instrument. There are some tools that it’s a pleasure to own, the ones that stay with you for life if you take care of them and which are passed down to your children and eventually as family heirlooms to theirs. It’s with regret that I’m handing back my borrowed micrometers to their owners, and it’s likely I’ll be making space for one of my own in my life before too long. If you understand this and you don’t own a Vernier caliper or a micrometer yet, then something tells me you too may be ordering one or the other pretty soon.